Copper Nanocrystals Encapsulated in Zr-based Metal–Organic Frameworks for Highly Selective CO₂ Hydrogenation to Methanol

We show that the activity and selectivity of Cu catalyst can be promoted by a Zr-based metal–organic framework (MOF), Zr₆O₄(OH)₄(BDC)₆ (BDC = 1,4-benzenedicarboxylate), UiO-66, to have a strong interaction with Zr oxide [Zr₆O₄(OH)₄(−CO₂)₁₂] secondary building units (SBUs) of the MOF for CO₂ hydrogenation to methanol. These interesting features are achieved by a catalyst composed of 18 nm single Cu nanocrystal (NC) encapsulated within single crystal UiO-66 (Cu⊂UiO-66). The performance of this catalyst construct exceeds the benchmark Cu/ZnO/Al₂O₃ catalyst and gives a steady 8-fold enhanced yield and 100% selectivity for methanol. The X-ray photoelectron spectroscopy data obtained on the surface of the catalyst show that Zr 3d binding energy is shifted toward lower oxidation state in the presence of Cu NC, suggesting that there is a strong interaction between Cu NC and Zr oxide SBUs of the MOF to make a highly active Cu catalyst

Plasmon-Enhanced Photocatalytic CO₂ Conversion within Metal–Organic Frameworks under Visible Light

Materials development for artificial photosynthesis, in particular, CO₂ reduction, has been under extensive efforts, ranging from inorganic semiconductors to molecular complexes. In this report, we demonstrate a metal–organic framework (MOF)-coated nanoparticle photocatalyst with enhanced CO₂ reduction activity and stability, which stems from having two different functional units for activity enhancement and catalytic stability combined together as a single construct. Covalently attaching a CO₂-to-CO conversion photocatalyst Re^I(CO)₃(BPYDC)­Cl, BPYDC = 2,2′-bipyridine-5,5′-dicarboxylate, to a zirconium MOF, UiO-67 (Re_<i>n</i>-MOF), prevents dimerization leading to deactivation. By systematically controlling its density in the framework (<i>n</i> = 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell), the highest photocatalytic activity was found for Re₃-MOF. Structural analysis of Re_<i>n</i>-MOFs suggests that a fine balance of proximity between photoactive centers is needed for cooperatively enhanced photocatalytic activity, where an optimum number of Re complexes per unit cell should reach the highest activity. Based on the structure–activity correlation of Re_<i>n</i>-MOFs, Re₃-MOF was coated onto Ag nanocubes (Ag⊂Re₃-MOF), which spatially confined photoactive Re centers to the intensified near-surface electric fields at the surface of Ag nanocubes, resulting in a 7-fold enhancement of CO₂-to-CO conversion under visible light with long-term stability maintained up to 48 h